An internal combustion engine comprises a number of cylinders, each of which has a piston, which slides cyclically inside the cylinder, and a spark plug controlled cyclically by an electronic central control unit to produce a spark between its electrodes and so ignite the compressed gas inside the cylinder. The central control unit comprises a memory containing a number of maps of spark plug control values as a function of the current engine status. More specifically, the maps give the spark lead value for each spark plug, i.e. the value of the angular interval between ignition, i.e. the spark between the spark plug electrodes, and the top dead center or TDC position of the piston. If the spark lead value is zero, then ignition, i.e. the spark between the spark plug electrodes, occurs precisely at the top dead center or TDC position of the piston.
The spark lead values memorized in the central control unit maps are determined at the engine tuning stage to ensure efficient combustion in all possible operating conditions, and good thermal efficiency of the engine, while at the same time safeguarding the engine itself, i.e. against excessive engine knock in the cylinders. Combustion, however, is affected by numerous factors (foremost of which being fuel characteristics, cylinder head temperature, and wear of the spark plugs) which are substantially impossible to predict accurately. For this reason, a knock sensor is used to determine the presence of or excessive engine knock in the cylinders. In the event of excessive engine knock in the cylinders, this is eliminated by the central control unit reducing the spark lead value with respect to the map value.
The central control unit controlling the spark plugs traditionally employs the memory map spark lead value, and reduces it in the event of excessive engine knock in the cylinders. This is a preventive control strategy, in that its sole object is to safeguard the engine by modifying the map spark lead value in the event of excessive engine knock in the cylinders.
It has recently been observed that a small amount of engine knock is other than negative, and in fact provides for improving combustion (and therefore maximizing thermal efficiency) without compromising the engine. A more aggressive spark lead control strategy has therefore been proposed, whereby the map spark lead value is not only modified in the event of excessive engine knock, but also to obtain a small amount of engine knock to optimize combustion in the cylinders. Typically, the more aggressive strategy provides for steadily increasing the map spark lead value until a predetermined maximum knock level is reached; at which point, the spark lead is reduced in steps to such a value as to eliminate knock, and is then increased steadily again.
Tests show that, as compared with the conventional preventive strategy, the aggressive strategy described provides for improving thermal efficiency of the engine and so increasing drive torque for a given fuel consumption (or reducing fuel consumption for a given drive torque). The same tests, however, also show the aggressive strategy to be only capable of maintaining optimum combustion for a few short periods, with the result that combustion efficiency on average is good but not excellent.
U.S. Pat. No. 4,711,212A1 discloses an anti-knocking control system for controlling an internal combustion engine to prevent knocking in accordance with a knocking condition detected by a knock detecting apparatus on the basis of a distribution pattern of knock magnitude values derived from the output signal of a knock sensor; the knock detection apparatus is arranged to derive a knock magnitude value from the output signal of the knock sensor generated within a predetermined engine rotational angle at an interval thereby obtaining a plurality of knock magnitude values, determine a pattern of distribution of the plurality of the knock magnitude values on the basis of the results of comparison between the frequency that the knock magnitude value exceeds an upper threshold value and the frequency that the knock magnitude value goes below a lower threshold value, and detect a knocking condition of the engine in accordance with the determined distribution pattern.